1. Field of the Invention
The present invention relates to an optical scanner that forms a latent image on an image carrier, an image forming apparatus including the same.
2. Description of the Related Art
In a currently used optical scanner and an image forming apparatus including the optical scanner, formation of a full color image, enhanced image quality, enhanced speed, and a low cost are increasingly called for. In the commonly used optical scanner, for meeting a demand of enhanced image quality, bending of scanning lines needs to be reduced, a fluctuation of a beam spot diameter on an image surface needs to be reduced, constant speed characteristics need to be enhanced, and a displacement of a beam spot needs to be reduced.
To increase the speed, multi-beam scanners that can simultaneously scan a plurality of the scanning lines has provided recently. However, in a currently used method, because a luminous point, which is a light source, is positioned at a distance from an optical axis in a sub-scanning direction, bent shapes of the scanning lines differ. An image forming apparatus such as a tandem image forming apparatus uses a full color compatible method in which different scanning optical systems scan scanned surfaces corresponding to a plurality of colors and resulting images are overlapped to form a full color image. Thus, due to the differing bent shapes of the scanning lines, using the multi beam method in the tandem image forming apparatus causes occurrence of color shift in the sub-scanning direction and the image quality is reduced.
Furthermore, if an interval differs for each image height due to bending of the scanning lines, a difference of grayscales occurs in the image and the image quality deteriorates.
The image forming apparatus that produces a suitable image under a normal environment may print out a color-shifted image because of a change in the room temperature or a rise in the temperature inside the image forming apparatus due to continuous printing. This is because such a temperature change causes a minute change in an optical element inside the image forming apparatus and causes a fluctuation in an optical path. A beam interval in the sub-scanning direction fluctuates and appears as color shift in the output image.
The drawback of color shift due to environmental fluctuations mentioned earlier is required to be overcome in the image forming apparatus, especially in the color image forming apparatus that attempts to further enhance a high density and a high quality of the image.
For realizing a lower cost while enhancing high image quality and high speed of the image forming apparatus, in the optical scanner that is disclosed in Japanese Patent Application Laid-open No. 2006-284822, the number of high-cost light sources is reduced to half and beam splitting elements are used that enable a high speed image output.
In a multi-beam light source, adjustment and accuracy of a beam spot position interval in the sub-scanning direction, in other words, adjustment and accuracy of a sub-scanning beam pitch is an important technology for obtaining a high quality image.
In a commonly used method, the light source is rotated around an axis that is orthogonal to the main-scanning direction and the sub-scanning direction to obtain the desired sub-scanning beam pitch. However, in a beam splitting method, because two photosensitive elements are exposed with lights emitted from a single light source, although the desired sub-scanning beam pitch can be obtained for one of the photosensitive elements, a pitch error occurs in the other photosensitive element due to a shape error or a fixing error of the optical elements other than the beam splitting elements.
To overcome the problem, a cylindrical-lens supporting method, which uses an intermediate member, is disclosed in Japanese Patent Application Laid-open No. 2006-284822.
There are two ways of supporting the cylindrical lens. One is called one-side supporting method and the other is called both-side supporting method. In the one-side supporting method, although an adjustment of the beam spot position interval in multiple directions is enabled, a shape of the intermediate member may change due to environmental fluctuations such as changes in temperature and humidity. If the shape of the intermediate member changes, the cylindrical lens is tilted and the beam spot position fluctuates, thus causing color shift in the output image.
Further, if the cylindrical lens is tilted, a wave aberration deteriorates, the beam spot diameter increases, and the image quality deteriorates.
In the both-side supporting method, although tilting of the cylindrical lens is less compared to the one-side supporting method, a degree of freedom in layout of the optical scanner decreases and the number of components increases. Thus, the both-side supporting method is inappropriate for producing a low-cost image forming apparatus.